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TTVs are derived from Q1-Q15 Kepler data.  x-axes: “Observed Tc” (Mid-Transit Time): EXOFAST’s best-fits from Kepler light flux vs. time data.  y-axes: “(O – C)”: difference between Observed Tc and the Calculated Tc from the graphically obtained linear ephemeris.  The plots are pictured in the order of orbital periods (smallest first).

Discussion:
Data from Q1-Q15 clearly shows a TTV anti-correlation between KOI-1831.03 and KOI-1831.01 (and their periods are near mean motion resonance of 3:2; observed: 3.03:2); the respective TTV_maximum and TTV_minimum are only slightly displaced by 14.66 days (753.24 vs. 767.80 days) and occur during Q9.  The P_ttv values and Lomb-Scargle periodogram main peak values do not agree well probably because the sinusoidal "best-fit" curves are each based on only ~ half of a frequency cycle.

1st: KOI-1831.02, P = 4.39 days [Plot error bars (smaller than symbol) = ± 8.63 min.]
No distinctive TTV (see plot).
Linear ephemeris: Tc = 4.38535441(Tc#) + 72.22429436

NO IMAGE: KOI-1831.04, P = 13.98 days
Too weak for good fits of individual transits; NEA-reported duration from folded transits = 1.398 hr., i.e., only 2-3 LC data points per transit; depth = 297 ppm.

2nd: KOI-1831.03, P = 34.21 days [Plot error bars (smaller than symbol) = ± 7.66 min.]
Online EXOFAST:
TTV_maximum: 753.24 ± 191.72 days, Amp_ttv_maximum: 161.25 ± 67.89 min.
The correlation coefficient, 0.9603, of the sinusoidal curve adds credibility to the follow values:
TTV_minimum: 1612.18 ± 319.76 days, Amp_ttv_minimum: -272.05 ± 67.89 min., and
P_ttv: 1717.89 ± 308.43 days.
Amp_ttv: 433.30 ± 96.01 minutes.
Lomb-Scargle periodogram, candidate P_ttv: 1384.27 days; Power: 10.93; P-value: 6.364 x 10^-11.
Linear ephemeris: Tc = 34.19365097(Tc#) + 81.92393469
Estimated P_ttv = 1/|(nmmri_a/P_a - nmmri_b/P_b)| = 1706.23 days

3rd: KOI-1831.01, P = 51.80 days [Plot error bars (smaller than symbol) = ± 4.39 min.]
Offline EXOFAST:
TTV_minimum: 767.80 ± 192.88 days, Amp_ttv_minimum: -36.55 ± 18.76 min.
The correlation coefficient, 0.9899, of the sinusoidal curve adds credibility to the following values:
TTV_maximum: 1668.57 ± 326.25 days, Amp_ttv_maximum: 74.47 ± 18.76 min., and
P_ttv: 1801.53 ± 319.99 days.
Amp_ttv: 111.02 ± 26.53 minutes.
Lomb-Scargle periodogram, candidate P_ttv: 1287.00 days; Power: 7.22; P-value: 6.468 x 10^-9.
Linear ephemeris: Tc = 51.80962091(Tc#) + 115.97660672

References:
• For Q1-Q6 TTV data of 1831.03 and 1831.01 (thru ~ 563 (BJD-2454900) days), see: Ford2012arXiv-1201.1892.
• P_ttv equation: P_ttv = 1/|(nmmri_a/P_a - nmmri_b/P_b)|…(see Fabrycky, et al., 2012, arXiv-1201.5415)

18 June 2013; updated 9 Aug 2013
KOI-1831 (Kepler-324, KIC-#11601584) 4-Planets