Ultra Long Period Cepheids: a primary standard candle up to the Hubble flow.

The cosmological distance ladder crucially depends on Classical Cepheids (CCs, with P=3-70d), which are primary distance indicators up to 25 Mpc. Within this volume, only a few SNe Ia have been calibrated through CCs, and even these carry uncertainties from the non-linearity and the metallicity dependence of the Period-Luminosity slope. Ultra- long period (ULP) Cepheids (P>100d) have recently been identified in star-forming galaxies. ULPs are very bright objects that are very promising candidates as standard candles that can be observed up to distance of 100 Mpc. Only few such objects have been discovered so far and their nature is not completely clear and need to be related to the metallicity and star-formation history of the host galaxy. Here we propose to obtain time-series GMOS-South imaging in r' and i' bands of CCs and ULPs in the BCD galaxy NGC1705 (Z=0.004). This compact galaxy, located at ~5 Mpc, can be properly observed only with GEMINI-South from ground. These observations will be critical to double the sample of known ULPs, in order to address their metallicity dependence and to better understand their nature, and possibly to extend the Cepheid distance scale calibration to D>300 Mpc (with E-ELT)