Ṁ–luminosity relations parametrised in terms of log L_bol and M_ini and fitted jointly for the four open clusters RSGC1, NGC 2100, NGC 7410, and χ Per. Left panel presents the best-fit to the Ṁ_SED measurements of the four clusters (Eq. (7)) while the right panel includes in the fit the Ṁ_CO-values of four stars in RSGC1 (purple downward triangles) using Eq. (8). Symbols, colours and shades have the same meaning as in Fig. 3, with the exception of the addition of the Ṁ_CO measurements derived in Sect. 3.1 using the L_bol values of Davies et al. (2008).