Seiji Ozawa has appointed two assistant conductors for the Boston Symphony Orchestra effective at the start of the 1998-99 season. Federico Cortese and Ilan Volkov succeed to the position currently held by Richard Westerfield.

Volkov studied at the Royal Academy of Music in London, the Rubin Academy of Music in Jerusalem, the Bachakadamie in Stuttgart, and the Dartington Summer School. Last year he was named principal conductor of the London Philharmonic Youth Orchestra.

The large audience for Andre Rieu and his Johann Strauss Orchestra, built by videos repeatedly shown during public television fund-raisers, will be gratified to know Don Law will present a concert by Rieu and his ensemble at the FleetCenter April 21. Many of Rieu's fans were disappointed that they could not get into his sold-out Symphony Hall concert last November. Tickets for the FleetCenter appearance go on sale Monday at 10 a.m. at the FleetCenter box office and through Ticketmaster.