We present observations with the adaptive optics assisted integral field spectrograph OASIS of the M 31 double nucleus in the spectral domain around the Calcium triplet at a spatial resolution better than 0 \aas@@fstack { \prime \prime } 5 FWHM .
These data are used to derive the two-dimensional stellar kinematics within the central 2″ .
Archival WFPC2 / HST images in the F300W , F555W and F814W bands are revisited to perform a photometric decomposition of the nuclear region .
We also present STIS / HST kinematics obtained from the archive .

The luminosity distribution of the central region is well separated into the respective contributions of the bulge , the nucleus including P1 and P2 , and the so-called UV peak .
We then show , using the OASIS kinematical maps , that the axis joining P1 and P2 , the two local surface brightness maxima , does not coincide with the kinematic major-axis , which is also the major-axis of the nuclear isophotes ( excluding P1 ) .
We also confirm that the velocity dispersion peak is offset by \sim 0 \aas@@fstack { \prime \prime } 2 from the UV peak , assumed to mark the location of the supermassive black hole .
The newly reduced STIS/HST velocity and dispersion profiles are then compared to OASIS and other published kinematics .
We find significant offsets with previously published data .
Simple parametric models are then built to successfully reconcile all the available kinematics .

We finally interpret the observations using new N-body simulations .
The nearly keplerian nuclear disk of M31 is subject to a natural m = 1 mode , with a very slow pattern speed ( 3 km/s/pc for M _ { BH } = 7 10 ^ { 7 } \mbox { M } _ { \sun } ) , that can be maintained during more than a thousand dynamical times .
The resulting morphology and kinematics of the mode can reproduce the M 31 nuclear-disk photometry and mean stellar velocity , including the observed asymmetries .
It requires a central mass concentration and a cold disk system representing between 20 and 40 % of its mass .
Such a slow mode could be excited when interstellar clouds from the more external gaseous disk infall towards the centre .
Nuclear disks formed from accreted gas are possible candidates for the precursors of these types of structure , and may be common in central regions of galaxies .