THERE IS SOMETHING vaguely repellant about the idea of operas with names like True Constancy or Fidelity Rewarded. They sound like they may be Morally Improving.

Perhaps that is why people don't usually think of Franz Joseph Haydn as a composer of operas; you look in a catalogue of his works, see that he composed operas named Le vera costanza and La fedelta premiata and you decide it might be more fun to hear his quartet named The Frog or The Horseman or the symphony named The Weirdo (well . . . II distratto; close enough). Another reason, I suppose, is that it is fairly easy to get four people together to do a string quarter - a lot easier than the complex logistics of soloists, orchestra, chorus, costumes and scenery required for opera. So, naturally, the quartets become better known.

Still, the fact is that Haydn wrote more operas than Puccini, and there is mounting evidence (making due allowance for diversities of taste) that some of them were better than Puccini's - comparable to those of the incomparable Mozart. The evidence has been placed in the capable hands of Antal Dorati, whohas already performed wonders with a complete recording of the Haydn symphonies for London Records. And now on Philips 6703 077 (three records), he makes the second installment of that evidence available to the rest of us in La vera constanza.

Inevitably, this opera, which has not been heard since 1791, makes a less startling first impression that last year's recording of La fedeltma premiata, because the territory is already partially explored. Fedelta is, marginally, a better opera, but Costanza, written a few years earlier, is the real turning point in Haydn's operatic career, marking a radical departure from the simplicity of his previous styles.

Tracing Haydn's development as an opera composer, we are plunged suddenly into a new world with La vera costanza; it is the world we know the best from the works of Mozart, complex and nuanced, with the serious and the comic mingled together and reinforcing one another's effects. It is a world that Haydn began to explore a few years before Mozart, though Mozart explored it, ultimately, in greaterdepth.

So startingly "MOzartian" is the musical atmosphere that one almost instinctively waits for an aria like Porgi, amor, particularly since the plot deals with amorous complications between a count and a woman named Rosina. An aria like that was still (just barely) beyond Haydn's reach, as it was also beyond Mozart's in 1778 when Haydn composed this opera. But the path from La vera costanza to The Marriage of Figaro is a straight one and fairly short. Haydn's dramatic power emerges most clearly in the long, complex finales to the first two acts of this opera, but it pervades the entire work - the subtly flexible recitatives, the varied, splendidly polished arias - giving the rather silly and pedestrian libretto a wholly unexpected and not entirely deserved depth of feeling.

World premiere recordings from the classical period become harder to produce each year. In its series of Haydn operas, Philips Records is pioneering in one of the few largely unexplored territories left in that period, and its first two explorations have struck real gold. Fortunately, since it is dealing with material not likely to be recorded again in the foreseable future, Philips made the necessary effort to produce a definitive recording the first time. Dorati conducts the Lausanne Chamber Orchestra, an ensemble expert in 18th-century style as well as in pure musicianship, and he brings to his work an impressive knowledge both of opera and of Haydn.

The leading roles are sung by Jessye Norman, who should need no introduction to Washington audiences, and Claes Ahnsjo, the splendid Swedish tenor who sang in The Creation during the 1975 Haydn Festival. The promise implied in the festival performance is splendidly fulfilled on this record; Ahnsjo's light, flexible and intelligently used voice should be heard in this country much more often than it is.

As for Miss Norman, her voice is in radiant form and Haydn's music suits it admirably. The role of Rosina demands the highest level of dramatic as well as vocal ability, and Jessye Norman rises splendidly to the challenge, finding real feeling in a rather contrived situation.

Except for Helen Donath, who has a fairly small role with one aria, the other singers are relative unknowns in this country, but they perform at a uniformly high level of quality with a superb sense of ensemble. Another triumph for the Haydn-Dorati combination.