Effective temperature as a function of the spectral type. For clarity the spectral types are not rounded. A second-order polynomial fit is shown. Possible outliers are removed by using a 2.5σ clipping procedure. The coefficients of the fit as well as the rms standard deviation are given in the plot. Data from Kenyon & Hartmann (1995, Table A.5) are overplotted using red filled circles, whilst green squares represent the median T_eff-spectral type sequence from Lépine et al. (2013).