Upper panel: selected estimates of the mass of the neutron star in Vela X-1 taken from the literature. The estimates are for specific assumed inclinations (see the original papers for how these where chosen) or plotted for a range of values with i = 90° as the lowest mass point and then 5° steps to i = 70°, to show the sin(i) dependence. See text for the multiple solutions provided by van Kerkwijk et al. (1995). The shaded areas on the right indicate possible maximum masses for neutron stars based on multi-messenger observations of GW170817 (see Margalit & Metzger 2017, and references therein). To set them into perspective, the lower panel shows the distribution of derived masses of other neutron stars in the literature taken from Alsing et al. (2018) for comparison.