Observed T_eff vs orbital period (P_orb) for donor stars in BH XRTs. Asterisks indicate ≈2 − 5 M_⊙ donor stars (IMXBs) while solid black circles low-mass ≲1.5 M_⊙ donors (LMXBs). The open circles mark the position of the early G-type donor in BW Cir and the nuclear-evolved companion in XTE J1118+480. Solid triangles in cyan indicate donor stars in CVs from the compilation of Knigge (2006). The blue open triangles indicate the three nuclear-evolved donors in EI Psc, QZ Ser and SDSS 1702+3229. The red line shows our empirical fit to the group of CVs and BH LMXBs, excluding BW Cir and the nuclear-evolved companions in XTE J1118+480 and the three CVs. For comparison, we also plot T_eff tracks of main sequence (MS), terminal age main sequence (TAMS) and stripped-giant stars that would fill the Roche lobes for a given P_orb.