Variation of the ratio of the maximum systemic radial velocity to the distance of an ellipsoid for 5 different values of its ellipticity (labeled in blue) with its major axis inclination. The ratio between the observed maximum systemic radial velocity of FH Ser and its Gaia distance is marked by a solid vertical line, with 1-σ uncertainty marked by vertical dotted lines and the 2-σ lower uncertainty by a vertical dashed line. The black dot corresponds to the best fit value.