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We present GTC-10.4m spectroscopy and multi-band photometry of the faint (r~26) optical counterpart of the anomalous X-ray pulsar 4U 0142+61. The 5000 -9000 Angs spectrum -- the first obtained for a magnetar -- is featureless,allowing us to set an equivalent width upper limit EW < 25 Angs to the presenceof emission lines in the Halfa region. Multi-band photometry in the g, r, i, zSDSS bands obtained at different epochs over 12 years shows no significantvariability from minutes-to-years time scales. The photometry has beencalibrated, for the first time, against the SDSS itself, resulting in solidupper limits to variability ranging from ~0.2 mag in i (over 12 years) to 0.05mag in z (over 1.5 years). The shape of the optical + near-infrared (literaturevalues) spectral energy distribution is not well constrained due to the highextinction along the line of sight. Using a Markov Chain Monte Carlo analysiswe find that it can be described by a power-law with a spectral indexbeta=-0.7+-0.5 and E_(B-V)=1.5+-0.4. We also discuss on the implications ofadding hard X-ray flux values from literature to the spectral fitting.

The featureless and non-variable optical spectral energy distribution of AXP 4U 0142+61