Black holes, for all their popularity as subjects of speculation, have never been proven to exist and, until now, only two objects in the heavens have been generally regarded as probable black holes.

Now astronomers report a third candidate, a mysterious, invisible point in the sky around which a faint, red star is orbiting rapidly, traveling more than 1 million miles per hour to complete a circuit every 7.8 hours. The red star, about two-thirds the size of the sun, is in the constellation Monoceros.

According to astronomical theory, black holes are the remnants of large stars that have exhausted their nuclear fuel, blown off their remaining lighter elements in a supernova and then collapsed to a dense core.

If the remaining mass of the star is small, it will condense into a pulsar, which still emits some light. But if the remaining mass is large, its gravity will pull the matter into a superdense point. The force of gravity becomes so strong that even light cannot escape.

The new black hole candidate was discovered when astronomers noticed that whenever its companion star flared, spewing out gases, there was a burst of X-rays from nearby. Astronomers believe that when matter falls into a black hole, it gives off high-energy X-rays before it disappears. The red star's tight, high-speed orbit also indicates it must be circling an object of unusually great mass.

The discovery was announced at an American Astronomical Society meeting by Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics and Ronald A. Remillard of the Massachusetts Institute of Technology.