Mass–radius relation for the companion of PSR J1544+4937 derived from different light curve modelings. The green circles are from the best hot spot model with T_spot = 4300 K, radius of 57°, centered at a latitude of 38° facing the neutron star, and viewed at i = 52°. The blue squares are from the best-fit hot dayside model with T_day = 4300 K and viewed at i = 51°. The red stars are from Icarus with 60°. Open and solid symbols denote models with distances of 5 kpc and 2.4 kpc, respectively. The error bars are derived from assumed neutron star masses of M_NS = 1.7 ± 0.3 M_☉. The black solid line shows a mean density of g cm⁻³, which is approximately the mean density of a Roche lobe filling star at an orbital period of P = 2.9 hr, and therefore is the lower limit of the companion of PSR J1544+4937. The other black curves show the theoretical mass–radius relations for brown dwarfs and helium and carbon objects. The dash-dotted lines are brown dwarfs with ages 0.1, 0.5, 1.0, and 5.0 Gyr, from top to bottom, from MESA (Paxton et al. 2011, 2013); the results are consistent with Chabrier et al. (2000) with extended coverage at lower masses. Dashed lines show helium objects with core temperatures of 5 × 10⁶, 10⁶, and 10² K, from top to bottom, and the dotted line shows carbon objects with core temperatures of 10⁶ (Deloye & Bildsten 2003).