Uncovering the host galaxy of the $\gamma$-ray-emitting narrow-line
  Seyfert 1 galaxy FBQS J1644+2619
  The discovery of $\gamma$-ray emission from radio-loud narrow-line Seyfert 1
(NLSy1) galaxies has questioned the need for large black hole masses (> 10$^8$
M$_{\odot}$) to launch relativistic jets. We present near-infrared data of the
$\gamma$-ray-emitting NLSy1 FBQS J1644+2619 that were collected using the
camera CIRCE (Canarias InfraRed Camera Experiment) at the 10.4-m Gran
Telescopio Canarias to investigate the structural properties of its host galaxy
and to infer the black hole mass. The 2D surface brightness profile is modelled
by the combination of a nuclear and a bulge component with a S\'ersic profile
with index $n$ = 3.7, indicative of an elliptical galaxy. The structural
parameters of the host are consistent with the correlations of effective radius
and surface brightness against absolute magnitude measured for elliptical
galaxies. From the bulge luminosity, we estimated a black hole mass of
(2.1$\pm$0.2) $\times$10$^8$ M$_{\odot}$, consistent with the values
characterizing radio-loud active galactic nuclei.
