IR excess analytic law for all star sample together with measurements. IR excess as derived from the SPIPS algorithm and represented by ad hoc analytic laws (Eq. (1)) for each star in the sample. For each photometric band, red dots with error bars are the mean excess value over the cycle of the Cepheid and the corresponding standard deviation. Red point with orange bars are photometric band discarded in this work (see Sect. 2.2). The green zone is the error on the magnitude obtained using the covariance matrix of SPIPS fitting result.