  Aims: In this study, we aim at investigating the properties of the active
longitude system of the young solar analogue HD 116956 in detail, especially
concentrating on determining the rotation period of the spot-generating
mechanism with respect to the photometric rotation period of the star itself.
Because the nonparametric approach, like the Kuiper method, can only give the
period of active longitudes, we formulate a new method that can determine the
parameters the active longitude distribution uniquely.
  Methods: For this purpose, we have developed an analysis method, based on
Bayesian statistics using Markov chain Monte Carlo, presented in this
manuscript. One of the advantages of this method is that an estimate of the
active longitude system rotation period, as well as the parameters of the shape
and location of the active longitudes together with their respective error
estimates. This allows us to compare the active longitude and mean photospheric
rotation periods of the star.
  Results: Our analysis confirms previous results of the object having two
stable active longitudes with a phase difference of Delta phi = 0.5, the other
longitude having dominated over the other one during almost the entire span of
the time series. Our method gives the rotation period of the active longitude
system P_al = 7.8412 +- 0.00002 d, which is significantly different from the
mean photospheric rotation period of the star P_rot = 7.817 +- 0.003 d.
  Conclusions: Our analysis indicates that the spot-generating mechanism,
manifesting itself as a system of two active longitudes, is lagging behind the
overall rotation of the star. This behaviour may be interpreted as a
nonaxisymmetric dynamo wave propagating in the rotational reference frame of
the stellar surface.
