Optical Identification of He White Dwarfs Orbiting Four Millisecond
  Pulsars in the Globular Cluster 47 Tucanae
  We used ultra-deep UV observations obtained with the Hubble Space Telescope
to search for optical companions to binary millisecond pulsars (MSPs) in the
globular cluster 47 Tucanae. We identified four new counterparts (to MSPs
47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs
47TucU and 47TucW). In the color magnitude diagram, the detected companions are
located in a region between the main sequence and the CO white dwarf cooling
sequences, consistent with the cooling tracks of He white dwarfs of mass
between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling
age, temperature and pulsar mass (as a function of the inclination angle) have
been derived and discussed. For 47TucU we also found that the past accretion
history likely proceeded in a sub-Eddington rate. The companion to the redback
47TucW is confirmed to be a non degenerate star, with properties particularly
similar to those observed for black widow systems. Two stars have been
identified within the 2-sigma astrometric uncertainty from the radio positions
of 47TucH and 47TucI, but the available data prevent us from firmly assessing
whether they are the true companions of these two MSPs.
