Period variations and long-term cyclic changes in the light curve for the very short-period K-type contact binary YZ Phe. (arXiv:1901.00237v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Sarotsakulchai_T/0/1/0/all/0/1">T. Sarotsakulchai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Qian_S/0/1/0/all/0/1">S.-B. Qian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soonthornthum_B/0/1/0/all/0/1">B. Soonthornthum</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_X/0/1/0/all/0/1">X. Zhou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_J/0/1/0/all/0/1">J. Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Li_L/0/1/0/all/0/1">L.-J. Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reichart_D/0/1/0/all/0/1">D. E. Reichart</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Haislip_J/0/1/0/all/0/1">J. B. Haislip</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kouprianov_V/0/1/0/all/0/1">V. V. Kouprianov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poshyachinda_S/0/1/0/all/0/1">S. Poshyachinda</a>

YZ Phe is a very short-period contact binary (Sp.= K2 V) with an orbital
period of 0.2347 days near the short period limit (0.22,d). Here we present
the complete light curves in $VRI$ bands, which photometric data were obtained
with the 0.61-m reflector of PROMPT-8 at CTIO in Chile during June to October
2016 and August 2017. The photometric solutions were determined by using the
W-D method and results reveal that YZ Phe is a W-subtype shallow contact binary
($f$ = 10%, q (W) = 2.635 or q = 0.379 in general meaning) with the variations
of size and location of spot on the more massive component, showing a strong
O’Connell effect with light-curve variations and long-term magnetic activity
cycles. By compiling all available eclipse times, the result shows a long-term
period decrease at a rate of $mathrm{d}P/mathrm{d}t = -1.32(pm 0.02)times
10^{-8}$ d $yr^{-1}$, superimposed on a cyclic variation ($A_3$ = 0.0081 days
and $P_3$ = 40.76 years). The cyclic change can be interpreted by the
light-travel time effect via the presence of a cool third body. Based on
photometric solutions, the third light was detected with 2% of total light in
V and I bands. Those support the existence of a third body. For the long-term
period change, it can be explained by mass transfer from the more massive
component ($M_2 sim 0.74 M_{odot}$) to the less massive one ($M_1 sim 0.28
M_{odot}$) or plus AML via magnetic braking. With q $<$ 0.4 and long-term period decrease, all suggest that YZ Phe is on the AML-controlled state and its fill-out factor will increase as well as the system will evolve into a deeper normal contact binary.

YZ Phe is a very short-period contact binary (Sp.= K2 V) with an orbital
period of 0.2347 days near the short period limit (0.22,d). Here we present
the complete light curves in $VRI$ bands, which photometric data were obtained
with the 0.61-m reflector of PROMPT-8 at CTIO in Chile during June to October
2016 and August 2017. The photometric solutions were determined by using the
W-D method and results reveal that YZ Phe is a W-subtype shallow contact binary
($f$ = 10%, q (W) = 2.635 or q = 0.379 in general meaning) with the variations
of size and location of spot on the more massive component, showing a strong
O’Connell effect with light-curve variations and long-term magnetic activity
cycles. By compiling all available eclipse times, the result shows a long-term
period decrease at a rate of $mathrm{d}P/mathrm{d}t = -1.32(pm 0.02)times
10^{-8}$ d $yr^{-1}$, superimposed on a cyclic variation ($A_3$ = 0.0081 days
and $P_3$ = 40.76 years). The cyclic change can be interpreted by the
light-travel time effect via the presence of a cool third body. Based on
photometric solutions, the third light was detected with 2% of total light in
V and I bands. Those support the existence of a third body. For the long-term
period change, it can be explained by mass transfer from the more massive
component ($M_2 sim 0.74 M_{odot}$) to the less massive one ($M_1 sim 0.28
M_{odot}$) or plus AML via magnetic braking. With q $<$ 0.4 and long-term
period decrease, all suggest that YZ Phe is on the AML-controlled state and its
fill-out factor will increase as well as the system will evolve into a deeper
normal contact binary.

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