  GJ 1214b is one of the few known transiting super-Earth-sized exoplanets with
a measured mass and radius. It orbits an M-dwarf, only 14.55 pc away, making it
a favorable candidate for follow-up studies. However, the composition of GJ
1214b's mysterious atmosphere has yet to be fully unveiled. Our goal is to
distinguish between the various proposed atmospheric models to explain the
properties of GJ 1214b: hydrogen-rich or hydrogen-He mix, or a heavy molecular
weight atmosphere with reflecting high clouds, as latest studies have
suggested. Wavelength-dependent planetary radii measurements from the transit
depths in the optical/NIR are the best tool to investigate the atmosphere of GJ
1214b. We present here (i) photometric transit observations with a narrow-band
filter centered on 2.14 microns and a broad-band I-Bessel filter centered on
0.8665 microns, and (ii) transmission spectroscopy in the H and K atmospheric
windows that cover three transits. The obtained photometric and
spectrophotometric time series were analyzed with MCMC simulations to measure
the planetary radii as a function of wavelength. We determined radii ratios of
0.1173 for I-Bessel and 0.11735 at 2.14 microns. Our measurements indicate a
flat transmission spectrum, in agreement with last atmospheric models that
favor featureless spectra with clouds and high molecular weight compositions.
