As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years .
Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole ( MBH ) with a mass of M _ { BH } =3-4 \times 10 ^ { 6 } M _ { \odot } 
implying an insignificant cusp mass .
However , it has also been shown that the central black hole resides in a \sim 1 ” diameter stellar cluster of a priori unknown mass .
In a spherical potential which is neither Keplerian nor harmonic , orbits will precess resulting in inclined rosetta shaped trajectories on the sky .
In this case , the assumption of non-Keplerian orbits is a more physical approach .
It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members .
This paper presents the first exemplary modelling efforts in this direction .
Using positional and radial data of star S2 , we find that there could exist an unobserved extended mass component of several 10 ^ { 5 } M _ { \odot } forming a so-called ’ cusp ’ centered on the black hole position .
Considering only the fraction of the cusp mass M _ { S 2 _ { apo } } within the apo-center of the S2 orbit we find as an upper limit that M _ { S 2 _ { apo } } / ( M _ { BH } + M _ { S 2 _ { apo } } ) \leq 0.05 .
A large extended cusp mass , if present , is unlikely to be composed of sub-solar mass constituents , but could be explained rather well by a cluster of high M/L stellar remnants , which we find to form a stable configuration .