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').