HRD positions of NGC 752 members and their companions. The NGC 752 members with UV excess and a binary fit are shown as red-filled squares, and hot companion candidates as blue-filled squares. Cyan crosses mark the X-ray-detected sources. Previously detected compact companions in the OCs King 2 (green diamonds), M67 (green squares), Melotte 66 (green star), NGC 188 (green circles), NGC 2506 (green downward pointing triangles), NGC 6971 (green upward pointing triangles), NGC 7142 (green leftward pointing triangles), and NGC 7789 (green rightward pointing triangles) are also shown. The cluster isochrone (grey curve), Bergeron et al. (1995) WD cooling curves (dashed grey curves), Althaus et al. (2013) WD cooling curves (dotted grey curves), and HRD positions of field sdBs (blue dots) and ELMs (red dots) are shown for reference.