  We used GTC instrument OSIRIS to obtain long-slit spectra in the optical
range (520-1040 nm) of the planetary host star WASP-43 (and a reference star)
during a full primary transit event and four partial transit observations. We
integrated the stellar flux of both stars in different wavelength regions
producing several light curves. We fitted transit models to these curves to
measure the star-to-planet radius ratio, Rp/Rs, across wavelength among other
physical parameters. We measure a Rp/Rs in the white light curve of
0.15988^{+0.00133}_{-0.00145}. We present a tentative detection of an excess in
the planet-to-star radius ratio around the Na I doublet (588.9 nm, 589.5 nm)
when compared to the nearby continuum at the 2.9-sigma level. We find no
significant excess of the measured planet-to-star radius ratio around the K I
doublet (766.5 nm, 769.9 nm) when compared to the nearby continuum. Combining
our observations with previous published epochs, we refine the estimation of
the orbital period. Using a linear ephemeris, we obtained a period of
P=0.81347385 +/- 1.5 x 10^{-7} days. Using a quadratic ephemeris, we obtained a
period of 0.81347688 +/- 8.6 x 10^{-7} days, and a change in this parameter of
dP/dt = -0.15 +/- 0.06 sec/year. As previous results, this hints to the orbital
decay of this planet although a timing analysis over several years needs to be
made in order to confirm this.
