tokens: blue dots represent the brightness change during the outburst, while the green dots refer to the brightness outside of the outburst. 3. Discussions and Conclusions During the outburst of J162117, the spectral observations showed some properties of CVs, i.e., the broad, two-peaked emission lines (FWHM corresponding to 1500 km s −1 ), , and He ii 4686. These are indications of the accretion on a white dwarf (Scaringi et al. 2016 ). Moreover, the strong rotational disturbance of the emission lines in the eclipse indicates the presence of a rapidly rotating disk (Thorstensen 2016 ). Therefore, after ruling of the merging of a contact binary as an explanation of the outburst of J162117, this event was attributed to a dwarf nova outburst (Scaringi et al. 2016 ; Kjurkchieva et al. 2017 ). However, in comparison with known CVs, J162117 shows several peculiarities (Kjurkchieva et al. 2017 ), namely a deeper eclipse at the outburst than at the quiescence state (by a factor 2.8) and outburst amplitudes at the lowest limit of dwarf nova eruptions. These properties reveal that J162117 is not a normal CV. The binary system was monitored photometrically from 2016 April 19 to September 14, using several small telescopes in the world. We were lucky to obtain several high-precision light curves before the outburst. As shown in Figure 2 , there are two shoulders around the primary minima in the B - and V -band light curves. The two minima show a clear U-type shape and that their bottoms are nearly flat. The color curves plotted in the figure also show a U-type shape around the primary minima, while the color curves are flat outside the eclipses. All of these properties suggest that there are no accreting disks around the white dwarf and thus, the white dwarf is not accreting material from the cool secondary at the pre-outburst quiescent state. Apart from the eclipse of the white dwarf component, the BVR light curves shown in Figure 2 are dominated by ellipsoidal variability in a binary star as well as the magnetic activity of the K-type secondary. As displayed in Figure 2 , some small flare-like events are visible in the B -band light curves, while these events are not seen in the V and R band light curves. These are general properties of optical flares (e.g., Qian et al. 2012 ). Moreover, as shown in Table 1 , the errors of the B -band magnitudes for J162117 and the comparison star are about ±0.006, indicating that those B -band flares are true. The light curves in the figure are asymmetrical, displaying the O’Connell effect. Meanwhile, the depth of the secondary minimum (at phase 0.5) is deeper than that of the primary one (at phase 0.0). These observed properties could be explained by the secondary being covered by dark spots near the inner Lagrange point L 1 where the mass is expected to flow onto the primary. The fact that regions near the L 1 point are heavily spotted has been observed in the pre-CV V471 Tau and in a few CVs (e.g., Hussain et al. 2006 ;
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O B-CelestialObject O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O B-Wavelength O B-Wavelength O O O O O O O O B-Wavelength O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O B-Wavelength I-Wavelength O O B-CelestialObject O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O B-Citation I-Citation I-Citation I-Citation O