Context : Classical novae are quite frequent in M 31 .
However , very few spectra of M31 novae have been studied to date , especially during the early decline phase .

Aims : Our aim is to study the photometric and spectral evolution of a M 31 nova event close to outburst .

Methods : Here , we present photometric and spectroscopic observations of M31N 2005-09c , a classical nova in the disk of M 31 , using the 1.3m telescope of the Skinakas Observatory in Crete ( Greece ) , starting on the 28th September , i.e .
about 5 days after outburst , and ending on the 5th October 2005 , i.e .
about 12 days after outburst .
We also have supplementary photometric observations from the La Sagra Observatory in Northern Andalucía , Spain , on September 29 and 30 , October 3 , 6 and 9 and November 1 , 2005 .
The wavelength range covered by the spectra is from 3565 Å to 8365 Å .
The spectra are of high S/N allowing the study of the evolution of the equivalent widths of the Balmer lines , as well as the identification of non-Balmer lines .

Results : The nova displays a typical early decline spectrum that is characterized by many weak Fe ii multiplet emissions .
It is classified as a P _ { fe } nova .
From the nova light curve , we have also derived its speed class , t _ { 2 } = 14 \pm 2.5 days .
As the nova evolved the Balmer lines became stronger and narrower .
The early decline of the expansion velocity of the nova follows a power law in time with an exponent of \simeq - 0.2 .

Conclusions :