def calc_mass_ratio_from_velrs(velr_1, velr_2):
    """Calculate ratio of stellar masses from observed radial velocities.
    Assumes circular orbit.
    
    Parameters
    ----------
    velr_1 : float
        Observed radial velocity of star 1. Unit is m/s.
    velr_2 : float
        Observed radial velocity of star 2. Unit is m/s.
    
    Returns
    -------
    mass_ratio : float
        Ratio of stellar masses for stars 1 and 2 as mass1 / mass2. Unitless.
        
    Notes
    -----
    m1 / m2 = a2 / a1, where a is semi-major axis of low-eccentricty orbit.
    v = 2*pi*a / P, where v is orbital velocity, P is orbital period.
      = vr / sin(i), where vr is observed radial orbital velocity, i is orbital inclination.
    => m1 / m2 = v2r / v1r
    From equation 7.5 in section 7.3 of [1]_.
    
    References
    ----------
    .. [1] Carroll and Ostlie, 2007, An Introduction to Modern Astrophysics

    """
    mass_ratio = velr_2 / velr_1
    return mass_ratio
