Hagit Halaf and Christoph Emmanuel Langheim - a violin/viola duo and a couple - will appear as soloists in Mozart's Sinfonia Concertante with the Ramat Gan Chamber Orchestra under Aviv Ron on April 23 in Bat Yam (03-508-0031) and April 25 in Ramat Gan (03-578-1688). The Israeli Halaf and German Langheim first met at the Tel Aviv Music Academy, where Langheim came to from his native Frankfurt for a year, and they later continued their studies in Vienna. The pair began playing together at Langhem's family events, and they now tour Europe; their debut at Carnegie Hall happened two years ago. In addition to their current work with important orchestras in Italy, they perform a classical repertoire, tangos by Piazzola, arrangements by Langheim, and pieces composed for them by such composers as Italy's Riccardo Joshua Moretti, America's John Alan Rose, Israel's Ronen Shapira, Ella Sherif and Daniel Galai. They also play Jewish music, which Langheim - a fluent Hebrew speaker - arranges and ardently promotes. The duo, which recently produced its first disc under the Romeo label, will return to Israel with a special program later this year.