tokens: polynomial to each run. 2.2 APO and prompt Additional observations were obtained on 2017 March 4, using the Agile photometer on the 3.5 m telescope at Apache Point Observatory, and on 2017 May 5 and 2017 May 6 using the 60 cm robotic Prompt8 telescope at CTIO. The APO observations consisted of 515 30 s integrations obtained through a BG39 filter, and the Prompt observations consisted of 253 and 300 30 s integrations obtained through a Baader Clear (C) filter. The APO and Prompt data were reduced following our standard procedures (see e.g. Chote et al. 2014 ; SZ16 ) to extract aperture photometry, with one exception: the polynomial fit to remove long-period trends was omitted as there was insufficient data to separate the effect of differential extinction from the long-period variability intrinsic to GW Lib. 2.3 Hubble Space Telescope Quasi-simultaneous Hubble Space Telescope (HST) far-ultraviolet (UV) spectroscopy of GW Lib was obtained using the Cosmic Origins Spectrograph (COS; Green et al. 2012 ) on the nights of 2017 August 31, September 6, and September 13, near the end of the NGTS observation window. The first two observations occurred during bad weather at Paranal, so simultaneous observations were not possible. The weather was clear during the final run, but GW Lib set past the NGTS elevation limit less than 2 min before the HST observations started. The time-tag COS data were reduced following the procedure from TO16 to create ultraviolet light curves. Data were binned into 5 s sub-spectra, which were then background-subtracted and the Ly α and OI airglow lines at 1216 and 1302 Å were masked. Each spectrum was then binned in the spectral direction to create individual photometric points with a 5 s cadence. We present limited results from this data set where they provide important context to the NGTS observations, but defer the full analysis to a future publication (Toloza et al. in preparation). 3 LONG PERIOD VARIABILITY Photometry demonstrating the long-period behaviour was extracted using a 2.5 pixel aperture and all calibrations. Points more than 5σ from the nightly mean (42 in total) were rejected, and then the data were smoothed with a running 40 min mean (roughly 190 points) to reduce the scatter introduced by noise and the 20 min signal. The resulting 34-week long light curve is presented in Fig. 1 . Figure 1. The full 7.5 month NGTS light curve of GW Lib, smoothed with a 40 min boxcar filter to emphasize the long-period behaviour. Each fortnight is offset vertically by 0.5 units, and the observation duty cycle for each fortnight is listed in parentheses below the start date. The regular gaps in the light curve are due to the day/night cycle; the gaps on February 1–2 and August 11–15 were due to technical issues; the remaining gaps are nights when NGTS was not operating due to cloud. Several nights are marked with indicative periods to highlight the detection of the previously observed long-period modulations. The time windows
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