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We present a new method to derive binary inclinations in quiescent black hole(BH) X-ray transients (XRTs), based on the depth of the trough (T) fromdouble-peaked Ha emission profiles arising in accretion discs. We find that theinclination angle (i) is linearly correlated with T in phase-averaged spectrawith sufficient orbital coverage (>~50 per cent) and spectral resolution,following i (deg)=93.5 x T +23.7. The correlation is caused by a combination ofline opacity and local broadening, where a leading (excess broadening)component scales with the de-projected velocity of the outer disc.Interestingly, such scaling allows to estimate the fundamental ratio M1/Porb bysimply resolving the intrinsic width of the double-peak profile. We apply theT-i correlation to derive binary inclinations for GRO J0422+32 and SwiftJ1357-0933, two BH XRTs where strong flickering activity has hindereddetermining their values through ellipsoidal fits to photometric light curves.Remarkably, the inclination derived for GRO J0422+32 (i=55.6+-4.1) implies a BHmass of 2.7+0.7-0.5 Msun thus placing it within the gap that separates BHs fromneutron stars. This result proves that low-mass BHs exist in nature andstrongly suggests that the so-called "mass gap" is mainly produced by lownumber statistics and possibly observational biases. On the other hand, we findthat Swift J1357-0933 contains a 10.9+1.7-1.6 Msun BH seen nearly edge on(i=87.4+2.6-5.6 deg). Such extreme inclination, however, should be treated withcaution since it relies on extrapolating the T-i correlation beyond i>~75 deg,where it has not yet been tested.

A correlation between Ha trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32