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KOI's (Blue)  or
K2 Objects (Green)

TTVs are derived from Q1-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-3680.01, P = 141.24 days [Plot avg. error bars = ± 0.61 min., smaller than symbols]

TTV_maximum: 619.59 ± 78.44 days, Amp_ttv_maximum: 1.81 ± 0.43 min.
TTV_minimum: 958.77 ± 90.17 days, Amp_ttv_minimum: -2.35 ± 0.43 min.
TTV_maximum: 1297.95 ± 104.69 days, Amp_ttv_maximum: 1.81 ± 0.43 min.
P_ttv: 678.35 ± 41.24 days.
Amp_ttv: 4.15 ± 0.61 minutes.
Lomb-Scargle periodogram, periodicity (P_ttv): 684.92 days; Power: 2.33; FAP: 0.0545.
Linear ephemeris (this work): Tc = [141.24187420 ± 0.00025596](Tc#) + [189.20544717 ± 0.00158818]

Numerous literature and major Tc#, Tc, and TTV tabulation references can be found on my "Summary" webpage following the table.

3 Dec 2014
                 Kepler KOI-3680 (KIC-9025971) 2-(or more?)-Planet System

TTV Evidence:
The presence of both a credible periodicity in the Lomb-Scargle Periodogram (LSP) and a corresponding pronounced one (P_ttv) in a sinusoidal distribution of [(O-C) vs. Time] ("TTVxy") data points is strongly suggestive of the presence of a second (possibly unseen, non-transiting) gravitationally-perturbing planetary object in a system.  This is the case for KOI-3680.01.