Companion radius as function of mass. The solid lines indicate the limits placed on the radius of the companion at different inclinations for a given neutron star mass. The mass-radius relation of M giants from the results by Dumm & Schild (1998) is indicated by the green dots. M5- and M6-type giants from this sample are represented by blue diamonds and black stars, respectively. The solid blue line represents the radius calculated from the gravitational acceleration log(g) as determined by Hinkle et al. (2006) for a given red giant mass, and the area shaded in light blue indicates the corresponding uncertainty. The area shaded in grey indicates the mass limits of the red giant as determined by Hinkle et al. (2006) through a spectroscopic analysis of the oxygen isotopic abundances of ¹⁷O and ¹⁶O.