SIMBAD references

We report on multiband photometric observations of four binary millisecond pulsars with the Gran Telescopio Canarias. The observations led to detection of binary companions to PSRs J1630+3734, J1741+1351, and J2042+0246 in the Sloan g', r', and i' bands. Their magnitudes in the r' band are ≃24.4, 24.4, and 24.0, respectively. We also set a 3σ upper limit on the brightness of the PSR J0557+1550 companion in the r' band of ≃25.6 mag. Combining the optical data with the radio timing measurements and white dwarf cooling models, we show that the detected companions are cool low-mass white dwarfs with temperatures and ages in the respective ranges of (4-7) x 10³ K and 2-5 Gyr. All the detected white dwarfs are found to likely have either pure hydrogen or mixed helium-hydrogen atmospheres.