INTRUDERS: The Incredible Visitations at Copley Woods By Budd Hopkins Random House. 223 pp. $17.95 COMMUNION: A True Story By Whitley Strieber Morrow 299 pp. $17.95 LIGHT YEARS: An Investigation Into the Extraterrestrial Experiences of Eduard Meier By Gary Kinder Atlantic Monthly Press/Morgan Entrekin 265 pp. $18.95

IN 1943, Judge Felix Frankfurter met with Jan Karski, a Pole who had been eyewitness to the systematic murder of Jews by the Nazis. After hearing Karski describe the holocaust, he told him that he didn't believe what he was saying. It was not that he thought Karski was lying, Frankfurter explained, simply that he found it impossible to believe the events Karski described had really taken place.

Budd Hopkins borrows this anecdote from Walter Laqueur's The Terrible Secret to use as an analogy in his introduction to Intruders, a report on his investigation of a woman's abduction by aliens. It's an apt analogy, not only for Hopkins' book but for two other recently published books as well, all of which describe some kind of encounter with some kind of alien and all of which seriously strain our credulity.

Hopkins, a well-known UFO researcher and author of a previous book on the subject, Missing Time, received a letter in 1983 from an Indiana woman he calls "Kathie Davis," who described a fragmented memory of having encountered strange, gray-faced creatures one summer night in her backyard. The letter led Hopkins into a 2 1/2-year investigation -- involving hypnosis, lie detection and soil analysis -- and to a rather startling conclusion: "Kathie Davis" and others are subjects in an elaborate breeding project, a genetic experiment being performed upon humans by a race of aliens.

Under hypnosis, according to Hopkins, "Kathie" recollected what the aliens had apparently made her forget -- a lifelong series of encounters leading up to the summer evening in 1983 when she observed (and was poisoned by radiation from) an egg-shaped craft that landed behind her home (it ruined the lawn). Many of her encounters involved painful medical experiments, the most intrusive being artificial inseminations and the subsequent abductions of the fetuses. She also recalled that her mother, sister and two young sons had similarly been abducted and experimented upon, suggesting to Hopkins that the "Davises" are subjects in a multi-generational analysis.

The case of "Kathie Davis" forms the central chapters of Intruders and is, Hopkins believes, "the most complex UFO case" ever. But her case is far from unique. Hopkins briefly describes several other reproductive experiments, involving both men and women and including at least one case of rape. And hundreds of others, he maintains, have reported visitation and abduction experiences for other apparent purposes.

One of these is Whitley Strieber, a well known writer of horror fiction, who in Communion gives an account of his own rather horrifying experience with aliens. In 1985, Strieber, a self-proclaimed skeptic, went through a radical personality change, becoming profoundly depressed. He then spontaneously recalled an abduction that involved blood sampling, a rectal exam and a brain operation. Subsequently, under hypnosis, he like "Kathie Davis" began to recollect a lifetime of encounters, recognizable before only as periods of missing time. During his adult life, he had been constantly on the run, moving for no good reason -- except a constant sense of dread -- from place to place. And Strieber's family, too, appears to be involved in the visitations -- certainly his son and perhaps his wife and father.

If these two cases show many similarities, the third -- the case of Eduard "Billy" Meier, discussed by reporter Gary Kinder in Light Years -- is strikingly different. Meier is an unemployed Swiss handyman with a sixth-grade education who in the 1970s began reporting visits -- not kidnappings -- involving a female alien, Semjase, from a star system, Pleiades, 500 light years away. Meier claims to have had more than 100 contacts. He was taken riding in the Pleiadian spaceship, during which Semjase taught him many lessons about her spiritually advanced civilization. Meier documented his contacts with thousands of pages of notes (notes about scientifically advanced matters a man of his education shouldn't be familar with) and -- most convincingly to many -- with the best quality photographs ever seen of alien spacecraft -- crisp, colorful daylight shots that seem to many experts to be authentic.

KINDER USES the unbelievable Meier case to make a larger point: Not since the first UFO sightings following World War II has there been a rigorous, skeptical inquiry into the evidence for alien visitation, despite thousands of unexplained sightings and despite what all three authors see as a historical pattern of closer and closer encounters. The problem, according to Kinder, is twofold. First, there are the lunatics -- those who claim to have harvested potatoes on Saturn, for example -- who make it difficult for any serious-minded adult to confess any interest in the subject whatsoever. But second and more distressing, Kinder writes, is the U.S. government's steadfast refusal to examine the evidence. If the evidence is all fraudulent, a really tough examination of it would have put the kibosh on UFOs and aliens long ago.

So what do we as readers make of what is presented here? As I see it, there are only three possible conclusions. First, these might simply be hoaxes -- stories made up out of whole cloth. Strieber, a gifted writer, is certainly capable of crafting a convincing narrative, and I have to confess that Communion gave me the chills. But Strieber has involved his family in this case, as well as personal friends and reputable psychiatrists whom he identifies by name. If it is a hoax, a lot of real people are party to it; the same is the case with the other two accounts. In addition, Meier is simply too ingenuous to deceive. Hopkins' would be the most difficult account to corroborate because no real names are used, but unless he is flat out lying all of the events seem to have witnesses.

Once one eliminates fraud as an explanation, the most palatable explanation for reports such as these is a mental aberration. Meier is strange; his rantings about spiritually advanced star systems have a schizoid quality to them. But Strieber and "Kathie Davis" don't seem crazy; in fact, mental illness was Strieber's preferred explanation, and he sought out a psychiatric diagnosis to no avail. The revelations under hypnosis are not particularly convincing (hypnosis is unreliable and does sometimes create memories), but if there is a hallucination occuring, the authors point out, it is a massive hallucination involving friends and family and hundreds of others only alluded to here.

The third possibility is, of course, unacceptable -- that is, that these events actually took place. All three authors -- especially Strieber -- seem to know that they're trying something risky here. All seem almost reconciled to not being believed. But while Kinder is objective and seems to have no particular investment in convincing the reader, there is a sort of desperation to Hopkins' and Strieber's books that is hard to dismiss out of hand. Yet what they're saying is in the end incredible. I'm left, with Judge Frankfurter, not able to believe. :: Wray Herbert is managing editor of Psychology Today magazine. He writes regularly for Book World about books on science and medicine.