Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.

Detection of periodicity in the broad-band non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios which could lead to quasi-periodic variations in blazar light curves. For example, orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying as such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of ∼ 9.2 year-long, and exceptionally well-sampled optical light curve of the BL Lac OJ 287. The study primarily uses the data from our own observations performed at the Mt. Suhora and Kraków Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill in some of the gaps in the data. The Lomb-Scargle Periodogram and the Weighted Wavelet Z-transform methods were employed to search for the possible QPOs in the resulting optical light curve of the source. Both the methods consistently yielded possible quasi-periodic signal around the periods of ∼ 400 and ∼ 800 days, the former one with a significance (over the underlying colored noise) of ≥ 99%. A number of likely explanations for such are discussed, with a preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports the previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.

Radial-velocity measurements and sine-curve fits to the orbital velocity variations are presented for the seventh set of ten close binary systems: V410 Aur, V523 Cas, QW Gem, V921 Her, V2357 Oph, V1130 Tau, HN UMa, HX UMa, HD 93917, NSV 223. All systems, but three (V523 Cas, HD 93917, NSV 223), were discovered photometrically by the Hipparcos mission. All systems are double-lined (SB2) binaries and all, but the detached, very close system V1130 Tau, are contact binaries. The broadeningfunction permitted improvement of the orbital elements for V523 Cas, which was the only system observed before for radial velocity variations. Spectroscopic/visual companions were detected for V410 Aur and HX UMa. Several of the studied systems are prime candidates for combined light and radial-velocity synthesis solutions.

Radial-velocity measurements and sine-curve fits to the orbital radial velocity variations are presented for the last eight close binary systems analyzed the same way as in the previous papers of this series: QX And, DY Cet, MR Del, HI Dra, DD Mon, V868 Mon, ER Ori, and Y Sex. For another seven systems (TT Cet, AA Cet, CW Lyn, V563 Lyr, CW Sge, LV Vir and MW Vir) phase coverage is insufficient to provide reliable orbits but radial velocities of individual components were measured. Observations of a few complicated systems observed throughout the DDO close-binary program are also presented; among them an especially interesting is the multiple system V857 Her which -in addition to the contact binary -very probably contains one or more subdwarf components of much earlier spectral type. All suspected binaries which were found to be most probably pulsating stars are briefly discussed in terms of mean radial velocities and projected rotation velocities (v sin i) as well as spectral type estimates. In

Radial-velocity measurements and sine-curve fits to the orbital velocity variations are presented for the sixth set of ten close binary systems: SV Cam, EE Cet, KR Com, V410 Cyg, GM Dra, V972 Her, ET Leo, FS Leo, V2388 Oph, II UMa. All systems except FS Leo are double-lined spectroscopic binaries. The type of FS Leo is unknown while SV Cam is a close, detached binary; all remaining systems are contact binaries. Eight binaries (all except SV Cam and V401 Cyg) are the recent photometric discoveries of the Hipparcos satellite project. Five systems, EE Cet, KR Com, V401 Cyg, V2388 Oph, II UMa, are members of visual/spectroscopic triple systems. We were able to observe EE Cet separate from its companion, but in the remaining four triple systems we could separate the spectral components only through the use of the broadening-function approach. Several of the studied systems are prime candidates for combined light and radial-velocity synthesis solutions.RS CVn-type system.Our double-lined (SB2) spectroscopic orbit is very well defined, with small errors of the orbital parameters. We note that determinations of the radial velocity semi-amplitude of the primary component, K 1 , agree well in the four existing solutions. Starting with Hiltner (1953) (as reanalyzed by Rainger et al. (1991)), Rainger et al. (1991), Pojmanski (1998) and the current, the results have been (in km s −1 ) : 121.7 ± 1.9, 122.3 ± 1.5, 118.5 ± 2.0, and 121.86 ± 0.76. The results of Pojmanski (1998) differ the most, but are still within the errors of the solutions. The center of mass velocity, V 0 , seems to show a secular progression, although the data of Pojmanski (1998) deviate from the trend. In the same order as before: −16.2 ± 1.4, −11.2 ± 1.2, −13.7 ± 1.5 and −9.13 ± 0.78 km s −1 . Noting that the observations were made in 1947, 1988, 1993 and 1997, the trend may be related to the motion about the third body with the period of about 65 -75 years and the semi-amplitude of about 2 km s −1 (Rainger et al. 1991).The results of Pojmanski (1998), although the first to reveal the motion of the secondary component, show deviations described above and, even more importantly, differ rather substantially from our results in the value of K 2 : 211.5 ± 5.5 versus our 190.17 ± 1.73 km s −1 . Possibly, the discrepancies are due to the rather complex reduction scheme of the previous study which involved successive removal of the two spectral signatures from individual spectra. This was entirely unnecessary in our case because the broadening functions that we analyzed were very well defined and radial velocities of both components could be measured with great ease. Figure 4 shows one of the broadening functions for SV Cam, in comparison with other systems analyzed in this paper. On the basis of the widths of the individual signatures in the broadening functions, and taking into consideration the instrumental broadening, we estimate the apparent rotation velocity of the components, V 1 sin i = 122 ± 10 km s −1 and V 2 sin i = 85 ± 8 km s −1 . Patkos & Hempelman...