  Wavefront sensing and control (WFSC) will play a key role in improving the
stability of future large segmented space telescopes while relaxing the
thermo-mechanical constraints on the observatory structure. Coupled with a
coronagraph to reject the light of an observed bright star, WFSC enables the
generation and stabilisation of a dark hole (DH) in the star image to perform
planet observations. While WFSC traditionally relies on a single wavefront
sensor (WFS) input to measure wavefront errors, the next generation of
instruments will require several WFSs to address aberrations with different
sets of spatial and temporal frequency contents. The multiple measurements
produced in such a way will then have to be combined and converted to commands
for deformable mirrors (DMs) to modify the wavefront subsequently. We
asynchronously operate a loop controlling the high-order modes digging a DH and
a control loop that uses the rejected light by a Lyot coronagraph with a
Zernike wavefront sensor to stabilize the low-order aberrations. Using the
HiCAT testbed with a segmented telescope aperture, we implement concurrent
operations and quantify the expected cross-talk between the two controllers. We
then present experiments that alternate high-order and low-order control loops
to identify and estimate their respective contributions. We show an efficient
combination of the high-order and low-order control loops, keeping a DH
contrast better than 5 x 10-8 over a 30 min experiment and stability
improvement by a factor of 1.5. In particular, we show a contrast gain of 1.5
at separations close to the DH inner working angle, thanks to the low-order
controller contribution. Concurrently digging a DH and using the light rejected
by a Lyot coronagraph to stabilize the wavefront is a promising path towards
exoplanet imaging and spectroscopy with future large space observatories.
