  The technique of line depth ratios (LDR) is one of the methods to determine
the effective temperature of a star. They are crucial in the spectroscopic
studies of variable stars like Cepheids since no simultaneous photometry is
usually available. A good number of LDR-temperature relations are already
available in the optical domain, here we want to expand the number of relations
available in the near-infrared in order to fully exploit the capabilities of
current and upcoming near-infrared spectrographs.
  We used 115 simultaneous spectroscopic observations in the optical and the
near-infrared for six Cepheids and optical line depth ratios to find new pairs
of lines sensitive to temperature and to calibrate LDR-temperature relations in
the near-infrared spectral range. We have derived 87 temperature calibrations
valid in the [4800-6500] K range of temperatures. The typical uncertainty for a
given relation is 60-70 K, and combining many of them provides a final
precision within 30-50 K. We found a discrepancy between temperatures derived
from optical or near-infrared LDR for pulsations phases close to phi ~ 0.0 and
we discuss the possible causes for these differences.
  Line depth ratios in the near-infrared will allow us to spectroscopically
investigate highly reddened Cepheids in the Galactic centre or in the far side
of the disk.
