Select the appropriate orbital elements, and the mass of the secondary. The reason for not choosing the mass of the primary is that with decreasing primary mass, the situation is asymptotically reached where the primary settles on a Keplerian orbit of a planet sized body around a massive secondary and its velocity amplitude would not not increase anymore. This would prevent proper fitting of primary velocity curves of larger amplitudes. If, after fitting, the corresponding velocity amplitude is desired, one uses the modelk function, e.g. PRINT, MODELK('A').