We report on multi-band 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 ^ { \prime } , r ^ { \prime } and i ^ { \prime } bands .
Their magnitudes in the r ^ { \prime } band are \approx 24.4 , 24.4 and 24.0 , respectively .
We also set a 3 \sigma upper limit on the brightness of the PSR J0557 + 1550 companion in the r ^ { \prime } band of \approx 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 ) \times 10 ^ { 3 } K and 2–5 Gyr .
All the detected white dwarfs are found to likely have either pure hydrogen or mixed helium-hydrogen atmospheres .