Letter from Thomas Gold to Dr Frank Drake, School of Physics, University of Sydney Sending Dr Drake 'the paper of three x-ray sources which are all below the equator and therefore could not be investigated from Arecibo. It seems to me that it would be a good thing to look in those regions with great care in the hope of making optical identifications with any radio sources that may be there' [A copy of the paper is not included with this letter]. 30 July 1965