The soft X-ray transient GU Mus has been detected by XMM-Newton in the quiescent state .
The source is very faint , with a 0.5-10.0 keV unabsorbed flux of \simeq 1.1 \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } .
The spectra is well fit by an absorbed powerlaw with a photon index of \alpha = 1.6 \pm 0.4 , close to the value seen when the source was in the low/hard state in Aug. 1991 .
From our observed luminosity , it seems unlikely that the quiescent state emission is dominated by coronal X-rays from the secondary .
The flux also appears to be in agreement with the ADAF model of BH-transients in quiescence .