Returns a/Rs for given parameters.

def a_over_Rs(P,R2,M2,M1=1,R1=1,planet=True):
    """
    Returns a/Rs for given parameters.
    """
    if planet:
        M2 *= REARTH/RSUN
        R2 *= MEARTH/MSUN
    return semimajor(P,M1+M2)*AU/(R1*RSUN)