Nature 486, 90–92 (2012); 10.1038/nature11062

In this Letter, I omitted to cite ref. 21 after ‘archival data’ in line 9 of page 90, after ‘spectroscopy’ in line 42 of page 90, and after ‘white dwarfs’ in line 14 of the Fig. 1 legend. The Keck Telescope spectra (and masses) of six globular cluster white dwarfs illustrated in Fig. 1b–g were published and analysed in ref. 21, and were reproduced in this Letter. These data were collected over multiple observing runs during 2005–2008 (Principal Investigator: R. M. Rich).