Four-dimensional greybody factors and the effective string

Recently Maldacena and Strominger found that the calculation of greybody factors for D = 5 black holes carrying three U(1) charges gives striking new evidence for their description as multiply wound effective strings. Here we show that a similar result holds for D = 4 black holes with four U(1) charges. In this case the effective string may be thought of as the triple intersection of the 5-branes in M theory compactified on T⁷.