SOME LOVE AFFAIRS keep going on and on and on. Mary Sano and G. Hoffman Soto certainly can attest to that. Sano's fascination with Isadora Duncan began 25 years ago when she started studying with Duncan protégé Mignon Garland. Today the Japanese-born artist is still one of the Bay Area's most committed Duncan dancers, dividing her time between Japan and the United States. Her South of Market studio has become a gathering place for not only Duncan devotees but also other artists who admire Duncan's spirit of independence and devotion to personal vision. One of them is Soto, who in 1973 became involved with another strong woman with a clear vision, Anna Halpern. Soto joins Sano and her six dancers for this Duncan birthday concert – the seventh annual Dionysian Festival – with his own eight-member Sotomotion ensemble. All music is played live, featuring pianist Aki Amai. Sat., 8 p.m.; Sun., 3 p.m., 245 Fifth St., Studio 314, S.F. $13-$16. (415) 357-1817, www.duncandance.org. (Rita Felciano)