The authors present results of a search for wide companions to halo subdwarfs of known metallicity. They have taken CCD I-band frames in order to detect faint stars at angular separations of 1″- 30″from the subdwarfs. The target sample includes over 1000 stars, and about 50% have already been observed. For a small subset of this sample (about 50 stars), astrometry of CCD images taken three years apart has allowed to identify a handful of common proper motion pairs. Spectroscopic observations of three of them have been made and show that one companion is a late K-type star and two are M-type. The radial velocity and overall characteristics confirm that in all likelihood these are very low mass secondaries of halo stars. The coolest companions found in this survey could be near the faint end of the halo main sequence.