Corrigendum: Properties of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies (Astronomy and Astrophysics (2015) 575 (A13) DOI: 10.1051/0004-6361/201424972)

We found an error in the signs of the values of αo reported in Table 8 of Foschini et al. (2015). The corrected values are reported here in Table 1. In addition, the part of the discussion in Sect. 4.3 relevant to the optical frequencies, should be revised as follows: The average optical spectral index of the present sample of RLNLS1s is αo = 1.0 ± 0.8 (median 0.8), in agreement with the previous surveys of RQNLS1 (Constantin & Shields 2003) and RLNLS1s (Komossa et al. 2006a; Yuan et al. 2008). A comparison with the optical slopes measured by Whalen et al. (2006) reveals similar slopes (particularly, Fig. 4 in Whalen et al. 2006), with some exceptions likely due to the source variability. For example, J1159+2838 changed from αo ≈ -0.19 to 0.04, and J1358+2658 switched from αo ≈ 0.45 to 0.97. vanden Berk et al. (2001) integrated the SDSS spectra of more than 2200 quasars and found an average αo ≈ 0.44. They also note that by using only the low-redshift sources, the optical spectral index becomes steeper (αo ≈ 0.65). Our average value (αo ≈ 1.0) seems to be in agreement with this trend. The concepts of the essay remained unaltered.