Dear colleagues, I have just finished analysing my unfiltered CCD observations of MASTER OT J055845.55+391533.4 obtained on 2023, Feb 22/23 at CBA Extremadura Observatory (Spain) using a 0.40-m f/5.1 telescope and SX-46 CCD camera under relatively clear skies. The resulting light curve clearly shows the presence of regular superhumps (4 in total) with an amplitude of 0.20 mag, establishing MASTER OT J055845.55+391533.4 as a new SU UMa-type dwarf nova. A period analysis, using the ANOVA, Generalized-Lomb-Scargle, PDM and Conditional Entropy methods (Peranso 3.0) yields a combined superhump period of 0.0553 +/- 0.0004d. The object was at mag CV = 14.4 on Feb 23rd. I will send my observations to AAVSO, CBA and VSNET for further analysis. Best regards Tonny --- Tonny Vanmunster CBA Belgium Observatory CBA Extremadura Observatory <http://www.cbabelgium.com> http://www.cbabelgium.com PERANSO: The Light Curve and Period Analysis Software http://www.peranso.com