  We have conducted optical and X-ray simultaneous observations of SWIFT
J1753.5-0127 with RXTE and ULTRACAM, while the system persisted in its
relatively bright low/hard state. In the cross-correlation function (CCF), we
find that the optical leads the X-rays by a few seconds with a broad negative
peak, and has a smaller positive peak at positive lags. This is markedly
different from what was seen for the similarly interesting system XTE
J1118+480, and the first time such a correlation function has been so clearly
measured. Furthermore, there appears to be a significant variation of the
correlation with X-ray energy. We suggest a physical scenario for its origin.
