  We present an analysis of the orbital motion of the four sub-stellar objects
orbiting HR8799. Our study relies on the published astrometric history of this
system augmented with an epoch obtained with the Project 1640 coronagraph +
Integral Field Spectrograph (IFS) installed at the Palomar Hale telescope. We
first focus on the intricacies associated with astrometric estimation using the
combination of an Extreme Adaptive Optics system (PALM-3000), a coronagraph and
an IFS. We introduce two new algorithms. The first one retrieves the stellar
focal plane position when the star is occulted by a coronagraphic stop. The
second one yields precise astrometric and spectro-photometric estimates of
faint point sources even when they are initially buried in the speckle noise.
The second part of our paper is devoted to studying orbital motion in this
system. In order to complement the orbital architectures discussed in the
literature, we determine an ensemble of likely Keplerian orbits for HR8799bcde,
using a Bayesian analysis with maximally vague priors regarding the overall
configuration of the system. While the astrometric history is currently too
scarce to formally rule out coplanarity, HR8799d appears to be misaligned with
respect to the most likely planes of HR8799bce orbits. This misalignment is
sufficient to question the strictly coplanar assumption made by various authors
when identifying a Laplace resonance as a potential architecture. Finally, we
establish a high likelihood that HR8799de have dynamical masses below 13 M_Jup
using a loose dynamical survival argument based on geometric close encounters.
We illustrate how future dynamical analyses will further constrain dynamical
masses in the entire system.
