Optical spectral observations of three binary millisecond pulsars

We present the results of optical spectroscopy of stellar companions to three binary millisecond pulsars, PSRs J0621\(+\)2514, J2317\(+\)1439 and J2302\(+\)4442, obtained with the Gran Telescopio Canarias. The spectrum of the J0621\(+\)2514 companion shows a blue continuum and prominent Balmer absorption lines. The latter are also resolved in the spectrum of the J2317\(+\)1439 companion, showing that both are DA-type white dwarfs. No spectral features are detected for the J2302\(+\)4442 companion, however, its broadband magnitudes and the spectral shape of the continuum emission imply that this is also a DA-type white dwarf. Based on the spectral analyses, we conclude that the companions of J0621\(+\)2514 and J2317\(+\)1439 are relatively hot, with effective temperatures $T_{\rm eff}$$=\(8600\)\pm\(200 and 9600\)\pm\(2000~K, respectively, while the J2302\)+\(4442 companion is significantly cooler, \)T_{\rm eff}$$