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TTVs are derived from Q0-Q17 Kepler data.  x-axes: “Observed Tc” (Mid-Transit Time): EXOFAST’s best-fits from Normalized PDCSAP_FLUX Kepler light flux vs. time (BJD_tdb - 2454900) data.  y-axes: “(O-C)”: difference between Observed Tc and the Calculated Tc from the graphically obtained linear ephemeris.

Figure: KOI-757.03, P = 6.25 days [Plot avg. error bars = ± 8.05 min.]
Linear ephemeris (this work): Tc = 6.25297244(Tc#) + 104.29892733

Figure: KOI-757.01, P = 16.07 days [Plot avg. error bars = ± 2.63 min.]
Linear ephemeris (this work): Tc = 16.06863959(Tc#) + 106.62278788

Figure: KOI-757.02, P = 41.19 days [Plot avg. error bars = ± 5.51 min.]
TTV_minimum: ~ 766 days, Amp_ttv_minimum: ~ -28 min.
P_ttv: ~ 5350 days.
Amp_ttv: ~ 514 minutes.
Lomb-Scargle periodogram, periodicity (P_ttv): 1533.29 days; Power: 8.78; FAP: 2.088 x 10^-6.
Linear ephemeris (this work): Tc = 41.19688454(Tc#) + 98.00936743

For literature references, including Tc#, Tc, and major TTV tabulations, see my "Summary" webpage.

25 July 2014
KOI-757 (Kepler-229, KIC-10910878) System

Discussion:
When less than one cycle of periodicity has been observed by Kepler, the P_ttv values derived from a best-fit sinusoidal curve and the corresponding Lomb-Scargle periodogram can be quite different…and even sometimes (from the former method) not very precise.  This is the case for KOI-757.02 (see plot and data below).  Clearly, this can only be remedied by longer-term observations…which may or may not be in the offing.