The Royal Opera's enforced exile from its home in Covent Garden is throwing up the opportunity to experience a number of interesting operatic rarities in concert format which, perhaps, wouldn't ever receive a full-blown staging in the normal course of events. One such rarity, which has been absent, at least from the British opera house, virtually since it was composed some 70 years ago, is Richard Strauss's Die agyptische Helena.

The long operatic career of Strauss (right) seems to fall into two distinct periods, at least in so far as English productions of his works go. Lots of airings for Elektra and Salome, and the very different, though no less masterful, Rosenkavalier that followed it; a few for Ariadne auf Naxos, and then not a lot of room for very much else, except perhaps his very last opera of all, Capriccio, which, in fact, receives a much-awaited revival at Glyndebourne this summer.

Die agyptische Helena is the penultimate collaboration Richard Strauss undertook with his long-term librettist Hugo von Hofmannsthal. It is, perhaps, also the least performed and least highly regarded of them. There can be fewer more learned and cerebral librettists in the entire history of opera than the aristocratic Hofmannsthal. The poet resumed his working relationship with Strauss, after a gap of some five years, in 1922, when they devised a ballet to Beethoven's Incidental Music to The Ruins of Athens. It was then that Hofmannsthal, already steeped in the world of ancient Greece, came up with a scenario about Helen of Troy, who is taken back by her husband, Menelaus, after the Trojan war - despite her adultery with Paris which had precipitated the whole conflict in the first place.

The ever genial and laid back Strauss was hoping for a light-hearted piece along the lines of Offenbach's La belle Helene, but, as usual, what he got from his librettist turned out somewhat differently - a strange and elliptical study, full of dense psychological and symbolic nuances. There is even a part for an all-knowing sea mussel which, in addition to being omniscient, also has the gift of human speech with a contralto voice!

So interesting though the scenario and the verbal texture of the libretto is, it also presents its fair share of difficulties. On one level, the plot is simple but also contrived, and in the second act the characters seem to have little life of their own. Can events in the mythological world of ancient Greece sustain interest over a two-and-a- quarter-hour timespan, especially in a concert performance only?

Well, as usual, Strauss's glorious music should carry the day for here he again tries something slightly different - a bel canto opera with arias, ensembles and great set-pieces. In addition, the orchestration is bright and brittle; lavish yet transparent.

The Royal Opera has, yet again, assembled an expert team led by the magnificent voice of Deborah Voight in the title role and with Thomas Moser as Menelaus. On top of that, the opera is conducted by the brilliant young Strauss specialist Christian Thielemann. Hear it now - it might take a very long time to come round again.

`Die agyptische Helena' is at the Royal Festival Hall, South Bank, SE1 (0171-960 4242) 22 and 25 May, 7.30pm