2013JAVSO..41...58D

The Blazhko effect in V1820 Orionis and its period were reported for the first time by Wils et al. (MNRAS, 368, 1757; 2006) from a data analysis of the Northern Sky Variability Survey. The results of additional V1820 Orionis observations over a time span of four years are presented herein. From the observed light curves, 73 pulsation maxima have been measured. The times of light maxima have been compared to ephemerides to obtain the (O-C) values. The Blazhko period (27.917 n 0.002 d) has been derived from light curve Fourier analysis and from ANOVA analyses of the (O-C) values and of magnitudes at maximum light (Mmax). During one Blazhko cycle, a hump in the ascending branch of the light curve was clearly identified and has also created a double maximum in the light curve. The frequency spectrum of the light curve, from a Fourier analysis with period04, has revealed triplet, quintuplet structures, and a second Blazhko weak modulation (period=34.72±0.01d). V1820 Orionis can be ranked as a strongly modulated star based on its observed amplitude and phase variations. The amplitude ratio of the largest triplet component to main pulsation component is quite large: 0.34.